1. Field of the Invention
The present invention relates to variable displacement compressors that are used in vehicle air conditioners. More particularly, the present invention relates to a variable displacement compressor equipped with a displacement control valve that controls the inclination of a swash plate.
2. Description of the Related Art
A typical variable displacement compressor has a swash plate tiltably supported on a rotary shaft. The inclination of the swash plate is controlled based on the difference between the pressure in a crank chamber and the pressure in the cylinder bores. The stroke of each piston is varied in accordance with the inclination of the swash plate. The displacement of the compressor is varied, accordingly. The compressor is provided with a discharge chamber that is connected to the crank chamber by a supply passage. A displacement control valve is located in the supply passage. The control valve controls the flow rate of refrigerant gas from the discharge chamber to the crank chamber thereby controlling the pressure in the crank chamber. Accordingly, the difference between the pressure in the crank chamber and the pressure in the cylinder bores is varied.
The control valve includes a valve body for controlling the opening of the supply passage and a solenoid for actuating the valve body. The solenoid is connected to a driver that is controlled by a controller. The controller causes the driver to selectively excite or de-excite the solenoid in accordance with conditions of the compressor such as cooling load. Exciting and de-exciting of the solenoid permit the valve body to control the opening of the supply passage. The flow rate of refrigerant gas from the discharge chamber to the crank chamber is controlled, accordingly.
De-exciting the solenoid of the control valve from an excited state generates an electromotive force based on the self-inductance of the solenoid. The electromotive force is oriented in a direction preventing the magnetic flux that passes through the solenoid from changing and is called a counter-electromotive force. If an excessive current is generated by the counter-electromotive force, the current applies an excessive load to the driver. This may result in the driver malfunctioning.
Accordingly, it is an objective of the present invention to provide a variable displacement compressor that prevents a current generated by a counter-electromotive force of the solenoid in the displacement control valve from being supplied to the driver.
To achieve the above objective, the compressor according to the present invention has a drive plate located in a crank chamber and tiltably mounted on a drive shaft and a piston operably coupled to the drive plate and located in a cylinder bore. The drive plate converts the rotation of the drive shaft to reciprocating movement of the piston in the cylinder bore. The piston compresses gas supplied to the cylinder bore from a suction chamber and discharges the compressed gas to a discharge chamber from the cylinder bore. The inclination of the drive plate is variable according to a difference between the pressure in the crank chamber and the pressure in the cylinder bore. The piston moves by a stroke determined by the inclination of the drive plate to control the displacement of the compressor. The compressor further includes means for adjusting the difference between the pressure in the crank chamber and the pressure in the cylinder bore. The adjusting means includes a gas passage for conducting gas used for adjusting the pressure and a control valve for adjusting the amount of the gas flowing in the gas passage. The control valve includes a valve body for adjusting the opening size of the gas passage and a solenoid selectively excited and de-excited based on a supply of electric current to actuate the valve body. The solenoid generates a counter-electromotive force based on the self-inductance of the solenoid when the solenoid is de-excited. A protector is connected in parallel with the solenoid to pass the current based on the counter-electromotive force generated in the solenoid through the protector.